In order to power various electric facilities, a switching power supply with variable output voltages, such as USB Power Delivery applications, is increasingly popular. To ensure safe and reliable operation of the switching power supply under various output voltages, it is desired that the switching power supply has different over power protection point respectively corresponding to different output voltages.
In the conventional, there are two manners for over power protection.
One manner is a traditional over power protection on a primary side, in which power information is acquired by detecting a current of the primary side. If the traditional over power protection on the primary side is applied to the switching power supply with variable output voltages, once a power protection point is set, the power protection point will be the same irrelevant of the output voltage, which does not meet the requirement that respective over power protection points are set based on respective output voltages.
Another manner is an over current protection on a secondary side, in which, power information is acquired by detecting an output current. If the over current protection on the secondary side is applied to the switching power supply providing variable output voltages, the requirement that respective over power protection points are set based on respective output voltages may be satisfied; however, there still are at least two drawbacks: (1) it lacks flexibility, since corresponding over power protection point will be restricted by different output voltages once the over current protection point on the secondary side is set; (2) a sampling resistor is applied to the over current protection on the secondary side to acquire current information in general, which will consume additional power compared to the over power protection on the primary side, and is a disadvantage for improving efficiency of the switching power supply.